diff --git a/openbsc/include/openbsc/gprs_sgsn.h b/openbsc/include/openbsc/gprs_sgsn.h
index d14c128..8cfa073 100644
--- a/openbsc/include/openbsc/gprs_sgsn.h
+++ b/openbsc/include/openbsc/gprs_sgsn.h
@@ -162,10 +162,13 @@
 };
 
 
+/* look up PDP context by MM context and NSAPI */
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_by_nsapi(const struct sgsn_mm_ctx *mm,
 					   uint8_t nsapi);
+/* look up PDP context by MM context and transaction ID */
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_by_tid(const struct sgsn_mm_ctx *mm,
 					 uint8_t tid);
+
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_alloc(struct sgsn_mm_ctx *mm,
 					uint8_t nsapi);
 void sgsn_pdp_ctx_free(struct sgsn_pdp_ctx *pdp);
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index bab34d8..d104c4e 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -141,6 +141,7 @@
 }
 
 
+/* look up PDP context by MM context and NSAPI */
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_by_nsapi(const struct sgsn_mm_ctx *mm,
 					   uint8_t nsapi)
 {
@@ -153,6 +154,7 @@
 	return NULL;
 }
 
+/* look up PDP context by MM context and transaction ID */
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_by_tid(const struct sgsn_mm_ctx *mm,
 					 uint8_t tid)
 {
@@ -165,7 +167,6 @@
 	return NULL;
 }
 
-
 struct sgsn_pdp_ctx *sgsn_pdp_ctx_alloc(struct sgsn_mm_ctx *mm,
 					uint8_t nsapi)
 {